1. Field of the Invention
The present invention relates to a semiconductor device laser processing method used in such steps as crystallization, annealing, etc. of a semiconductor material. In particular, the present invention relates to a semiconductor device laser processing method used in a manufacturing method of a liquid crystal display device that uses thin-film transistors.
2. Description of the Related Art
In active matrix liquid crystal display devices, there is known a configuration in which a pixel area for displaying a video image and peripheral circuits for driving pixels are integrated on a single transparent substrate. In general, a glass substrate is used for a liquid crystal display.
In the pixel area, a large number of pixels are arranged in a matrix form and thin-film transistors are connected to the respective pixels. Constituted of thin-film transistors, the peripheral circuits control values of currents to flow through the thin-film transistors connected to the respective pixels.
In the pixel area, each pixel has a role of holding information that is sent from the peripheral driving circuits, and cannot attain that role unless the thin-film transistor connected to the pixel has a sufficiently small off-current. Further, if the off-current of the thin-film transistor largely varies from one pixel to another, even when receiving the same information from the peripheral driving circuits, the pixels display it differently. On the other hand, in the peripheral circuits, the thin-film transistor is required to have a large mobility. With a larger mobility, the circuit configuration can be made simpler and the display device is allowed to operate faster.
As described above, different characteristics are required for thin-film transistors in the peripheral circuits and those in the pixel area, even if they are formed on the same substrate. Thin-film transistors in the pixel area are not required to have a large mobility, but are required to have small off-currents that are uniform in the pixel area. Conversely, as for thin-film transistors in the peripheral circuits, the mobility has priority over the off-current characteristic; that is, they are required to have a large mobility.
In recent years, mainly for the following reason, extensive studies have been made of the temperature reduction in semiconductor device manufacturing processes. In liquid crystal electro-optical devices, the display portion needs to transmit light, and therefore needs to use a glass substrate, which is inexpensive and highly workable. However, glass substrates cannot withstand a high-temperature (1,000.degree. C. or higher) heat treatment. Therefore, the conventional IC manufacturing technology using a silicon wafer cannot be applied directly to liquid crystal electro-optical devices.
The techniques in thin-film transistor manufacturing processes which should attain temperature reduction are techniques for improving the thin-film transistor characteristics.
(1) Crystallizing amorphous components included in a semiconductor material or an amorphous semiconductor material itself.
(2) Recovering the crystallinity of a semiconductor material that was originally crystalline but has been lowered in crystallinity by ion implantation.
(3) Improving the crystallinity of a semiconductor material that is insufficient in crystallinity.
Conventionally, thermal annealing is employed for the above purposes. That is, where silicon is used as a semiconductor material, crystallization of an amorphous material, recovering of crystallinity, improvement of crystallinity, and the like are performed by annealing a subject material at 600 to 1,100.degree. C. for 0.1 to 48 hours.
In general, as the thermal annealing is performed at a higher temperature, the processing time can be made shorter and the crystallization effects become remarkable. However, at a temperature lower than 500.degree. C., the thermal annealing has almost no effect. Therefore, from the viewpoint of the temperature reduction of a process, it is now necessary to replace a thermal annealing step with a step that is based on another technique.
The annealing technique of laser light illumination is now attracting much attention as a technique for replacing the thermal annealing. This is because the laser annealing can apply, in the form of laser light, as high energy as in the case of the thermal annealing to a restricted portion which requires annealing; that is, it is not necessary to expose the entire substrate to a high-temperature atmosphere.
Generally two methods of laser light illumination have been proposed. In a first method, a spot-like beam is applied to a semiconductor material by using a CW laser such as an argon ion laser. When a spot-like beam is applied to a semiconductor material, due to a difference in the energy profile of the beam and movement of the beam, the semiconductor material is melted and then gradually solidified, i.e., crystallized.
In a second method, a pulsed oscillation laser such as an excimer laser is used. A pulse laser light of a high energy density is applied to a semiconductor material, whereupon the semiconductor material is melted instantaneously and then solidified, i.e., crystallized.